OK, that's interesting and some of it checks out, although I'm very picky about etymology. I did not know that the symbol for Saturn was based on "KP" for Kronos and merged with the extant scythe symbol, but that is backed up by Salmasius. We also know that Christ was represented by the chi-rho "XP" and variations. But cross, from Latin crux, comes from neither of these because C took the place of gamma in Greek, and CR was never used as an abbreviation for cross, and I don't think a theory that velar plosives followed by liquids have general mergence power would hold water.

I see that adding a crossbar to the KP scythe happened much later (same as adding a crossbar to IHC). It's more accurate to say that the devil is always blurring words and mixing etymologies to make the present look more like it's connected to his strands in the past rather than others; but a symbol is what the individual makes of it, and so to the pure all symbols are pure. The history of the dollar sign includes at least eight identifiable strands.

So I'd go with you on the second reaper (whom I identify as Auriga) being possibly Kronos with the scythe for the harvest of grapes. To your other questions, my link pointed out that the Christian cross builds on the proto-Semitic taw and the Nehushtan serpent, which are very old and point back to Genesis 3 as well, so it has an established divine line of symbolism that has never broken; similarly "Christ" (from the Greek word for anointing, Hebrew "Mashiach") is an established title by the time Jesus comes. Kronos, like Uranus and the occasional Greek identification with Chronos, is a corrupted version of the Father, as Zeus is a corrupted version of the Son. The abbreviation XP is much earlier than and not directly related to KP, though the development process is similar. So it could be said that XP (chi cross) and the scythe were both earlier exploitable symbols and KP was an attempt to merge these.

If we take Revelation as accurate 1st-century symbolism it clearly teaches that there are two users of the scythe and these are not merged (as the Grim Reaper symbol today merges them). There is no evidence that the use of XP or cross for one and the use of the scythe for the other were merged by or during the important syncretistic period in the 4th century. It seems that when the merger happened it would involve both your hypotheses, the Catholic segment of the church adopting the pagan scythe as if conquering it, and pagans infiltrating the later church as if worshipping Christ as if the pagan scythe.

It's true that some have trouble with this. Feel free to draw conclusions anywhere, or not, but the broader and more superficial the conclusion, the likelier it is to get challenged and the more superficial the challenge from others is likely to be. The truth of Jesus always has older roots than all rivals, which are always counterfeits.